archive: add WithSourceDateEpoch() for whiteouts

This makes diff archives to be reproducible.

The value is expected to be passed from CLI applications via the $SOUCE_DATE_EPOCH env var.

See https://reproducible-builds.org/docs/source-date-epoch/
for the $SOURCE_DATE_EPOCH specification.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

pkg/epoch/epoch.go

@@ -0,0 +1,69 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+// Package epoch provides SOURCE_DATE_EPOCH utilities.
+package epoch
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"time"
+
+	"github.com/sirupsen/logrus"
+)
+
+// SourceDateEpochEnv is the SOURCE_DATE_EPOCH env var.
+// See https://reproducible-builds.org/docs/source-date-epoch/
+const SourceDateEpochEnv = "SOURCE_DATE_EPOCH"
+
+// SourceDateEpoch returns the SOURCE_DATE_EPOCH env var as *time.Time.
+// If the env var is not set, SourceDateEpoch returns nil without an error.
+func SourceDateEpoch() (*time.Time, error) {
+	v, ok := os.LookupEnv(SourceDateEpochEnv)
+	if !ok {
+		return nil, nil // not an error
+	}
+	i64, err := strconv.ParseInt(v, 10, 64)
+	if err != nil {
+		return nil, fmt.Errorf("invalid %s value %q: %w", SourceDateEpochEnv, v, err)
+	}
+	unix := time.Unix(i64, 0)
+	return &unix, nil
+}
+
+// SourceDateEpochOrNow returns the SOURCE_DATE_EPOCH time if available,
+// otherwise returns the current time.
+func SourceDateEpochOrNow() time.Time {
+	epoch, err := SourceDateEpoch()
+	if err != nil {
+		logrus.WithError(err).Warnf("Invalid %s", SourceDateEpochEnv)
+	}
+	if epoch != nil {
+		return *epoch
+	}
+	return time.Now()
+}
+
+// SetSourceDateEpoch sets the SOURCE_DATE_EPOCH env var.
+func SetSourceDateEpoch(tm time.Time) {
+	os.Setenv(SourceDateEpochEnv, fmt.Sprintf("%d", tm.Unix()))
+}
+
+// UnsetSourceDateEpoch unsets the SOURCE_DATE_EPOCH env var.
+func UnsetSourceDateEpoch() {
+	os.Unsetenv(SourceDateEpochEnv)
+}